300MHz-to-450MHz Low-Power, Crystal-Based

ASK Transmitter

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Crystal Oscillator

The crystal oscillator in the MAX1472 is designed to

present a capacitance of approximately 3.1pF between

the XTAL1 and XTAL2 pins. If a crystal designed to

oscillate with a different load capacitance is used, the

crystal is pulled away from its intended operating fre-

quency, thus introducing an error in the reference fre-

quency. Crystals designed to operate with higher

differential load capacitance always pull the reference

frequency higher. For example, a 9.84375MHz crystal

designed to operate with a 10pF load capacitance

oscillates at 9.84688MHz with the MAX1472, causing

the transmitter to be transmitting at 315.1MHz rather

than 315.0MHz, an error of about 100kHz, or 320ppm.

In actuality, the oscillator pulls every crystal. The crys-

tal’s natural frequency is really below its specified fre-

quency, but when loaded with the specified load

capacitance, the crystal is pulled and oscillates at its

specified frequency. This pulling is already accounted

for in the specification of the load capacitance.

Additional pulling can be calculated if the electrical

parameters of the crystal are known. The frequency

pulling is given by:

where:

Output Matching to 50

Ω

When matched to a 50

Ω system, the MAX1472 PA is

capable of delivering more than +10dBm of output

drain transistor that requires external impedance

matching and pullup inductance for proper biasing.

main purposes: It resonates the capacitance of the PA

output, provides biasing for the PA, and becomes a

high-frequency choke to reduce the RF energy cou-

work topology is shown in the Typical Application

Circuit. The matching network transforms the 50

Ω load

to a higher impedance at the output of the PA in addi-

tion to forming a bandpass filter that provides attenua-

tion for the higher order harmonics.

Output Matching to PC Board Loop

Antenna

In most applications, the MAX1472 PA output has to be

impedance matched to a small-loop antenna. The

antenna is usually fabricated out of a copper trace on a

PC board in a rectangular, circular, or square pattern.

The antenna has an impedance that consists of a lossy

component and a radiative component. To achieve

high radiating efficiency, the radiative component

should be as high as possible, while minimizing the

lossy component. In addition, the loop antenna has an

inherent loop inductance associated with it (assuming

the antenna is terminated to ground). For example, in a

typical application, the radiative impedance is less than

0.5

Ω, the lossy impedance is less than 0.7Ω, and the

inductance is approximately 50nH to 100nH.

The objective of the matching network is to match the

PA output to the small loop antenna. The matching

components thus transform the low radiative and resis-

tive parts of the antenna into the much higher value of

the PA output, which gives higher efficiency. The low

radiative and lossy components of the small loop anten-

na result in a higher Q matching network than the 50

Ω

network; thus, the harmonics are lower.

f

C

CCCC

p

m

case

load

case

spec

=

+

−

+

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⎜

⎞

⎠

⎟ ×

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